THE EFFECT OF WATER VAPOR ON COUNTERFLOW DIFFUSION FLAMES

THE EFFECT OF WATER VAPOR ON COUNTERFLOW DIFFUSION FLAMES by Jaeil Suh and Arvind Atreya Combustion and Heat Tkansfer Laboratory Department of Mehanial Engineering and Applied Mehanis The University of Mihigan Ann Arbor, MI 4819-2125 International Conferene on Fire Researh and Engineering, September 1-15, 1995 Orlando, FL Proeedings Sponsored by National Institute of Standards and Tehnology (NIST) and Soiety of Fire Protetion Engineers (SFPE) D Peter Lund and Elizabeth& Angell, IMitors Soiety of Fire Protetion Engineers, Boston, N@ 1995 NOTE This paper is a ontribution of the National Institute of Standards and Tehnology and is not subjet to opyright

THE EFFECT OF WATER VAPOR ON COUNTERFLOW DIFFUSION FLAMES JAEILSUH AND ARVIND ATREYA Combustion and Heat Transfer Laboratory Department of Mehanial Engineering and Applied Mehanis The University of Mihigan, Ann Arbo MI 4819-2125 Abstrat The hemial and physial effet of water vapor on the struture of ounterflow diffusion flames is investigated both eperimentally and theoretially The eperimental flame struture measurements onsist of profiles of temperature, stable gases and hydroarbons, soot and OH radial onentrations and spatially resolved radiative emission measurements These eperimental measurements are ompared with numerial alulations with detailed C2hemistry For these omputations, eperimentally measured temperature profiles were used instead of the energy equation to more aurately desribe the flame radiative heat losses The flame struture results show that as the water vapor onentration is inreased, the OH radial onentration inreases This inreases the flame temperature and the C2 prodution rate and dereases the CO prodution rate However, after approimately 3% water vapor substitution, the hemial enhanement by water vapor is not observed and the flame temperature begins to derease Introdution Water has been, and is, the most important fire suppression agent Currently, even though we have several other effetive fire suppression agents, water is the most prevalent and the only agent for large fires beause of its easy aessibility and non-toiity Water is supposed to behave as an inert in a fire, whereas, many hemial fire suppression agents are known to produe toi ompounds whih restrits their usage However, a lot of water is typially used to suppress a fire whih auses severe water damage (sometime even more severe than the fire damage) To limit the water damage and to minimize the water usage, it is important to understand the mehanisms of fire suppression by water Reently, there is onsiderable enthusiasm to use water mist as a substitute for halons and to redue the water damage Thk, however, requires the knowledge of adequate amount of water to efiliently suppress the fire Thus, there is a need to quantify the effet of water on flames Even though water has been used as a suppression agent for a long time, the eat mehanisms of fire suppression by water are not well understood Water is known to have two physial effets: (i) ooling of the burning solid by water evaporation and (ii) smothering aused by dilution of the oidizer and/or the fuel by water vapor These effets lead to fire suppression when water is applied to the fire However, in addition to these effets, another effet of water whih is not well known was observed and reently reported [1] This effet is the enhanement of hemial reations inside thej!ames with water vapor Transient eperimental results of referene [2] show an inrease in the flame temperature, C2 prodution rate and 2 depietion rate and a derease in the CO and soot prodution rate with water substitution (fuel and oidizer onentrations were held onstant) These results are different from C2 substitution whih redued the flame temperatures and suppressed the fire Thus, water substitution eperiments suggest that the hemial reations inside the flames are enhaned by water vapor In this paper, detailed struture of ounterflow diffusion flames with water vapor is measured and alulated to investigate what ours inside the flame and how the reations are enhaned The ounter- 13

flow flame onfiguration was hosen beause it represents the loal behavior of large turbulent diffusion flames typial of fires Measured temperature profiles were used for alulation to desribe the flame radiation heat losses more aurately and the alulations were performed with the full C2 mehanism Flame Temperature Measurements For flame temperature measurement, the ounterflow diffusion flame apparatus was used Shemati of this apparatus is shown in Figure 1 The gap between the fuel side and the oidizer side was 26mm and the radius of fuel and oidizer eit was 381 mm and 635mm respetively The flow rate of the fuel with diluent (nitrogen) through the fuel eit was 2 liter-per-minute, while that of the oidizer with two different diluents (nitrogen and argon) was 8 liter-per-minute The input onentration on the fuel side was 75% CI-14and 25% N2 and it was maintained onstant for various water substitution to the oidizer side flow The input onentration on the oidizer side was hanged as water vapor was substituted, holding the molar onentration of 2 onstant at 27 To maintain the same flow field and the same heat apait y of the oidizer flow, a miture of water vapor and argon was substituted for nitrogen This maintained the same molar flow rate and roughly the same speifi heat Therefore the amount of oygen whih flowed into the flame was the same all the eperiments (1, 2, 3 and 47 of water vapor substitutions) The flame temperature profile was measured with a oated S-type thermoouple (platinum and platinum with 1% rhodium) Silione dioide (Si2) oating was added and the measured temperature was orreted for radiation with simple alulation[3] Computational Method Numerial modeling of the hemial proess is performed using the Sandia Chemkin-based opposedflow dif/imion flame ode[4] The flame is modeled as a steady state opposed o-aial flow diffusion flame using the eperimentally measured enter line temperature profile Pressure is assumed to be onstant at 1 atm The reation mehanism for methane is C2 -full mehanism whih is onsist of 177 hemial reations with 32 speies More hemial reations will be added later to aount for hemial enhanement due to soot disappearane Governing Equations Starting with the Navier-Stokes equations in ylindrial oordinates, stream funtion in the form Y (, r) - r2u () is introduel[5] For laminar stagnation point flow in ylindrial oordinates the mass Stagnation Plane pdl I t OXIDIZER FIGURE 1 Shemati of the Counterflow Diffusion Flame Apparatus 14

onservation equation is epressed as follow $(rpu) +$(rpv) = O (1), Stream funtion, Y, satisfies this equation if it is defined as ZNJ = rpu = 2rU ar ay zdu Z=-rpv=rz from equations (2) and (3) it is lear that the aial veloity u depends only on and the radial veloity v varies linearly in r u= z (4),, = -:!! (5) P pd Assuming the temperature, T, and speies mass frations yk to be funtions of only and introduing these assumptions into the momentum equations without buoyant y, redues the partial differential equations to third o;der ordinary differential equations, viz (2) (3) :=~u$(:)-z~f(;:) +;fi[,:(;)+~q (6) ;:=;(?:)-;EJ-:[~:(;#j (7) From the right hand side terms of equations (6) and (7), it is also lear that both ~ 1 ap and ;( ) are funtions of alone Thus, ap is derived and the only possibility whih satisfy equation(8) is that = onstant This is denoted by H, u the radial pressure-gradient eigenvalue Here, a new variable G is also {ntrodued for onveniene in equation(7) du G() =, After substituting G() into equation(7) and using k-speies onservation equations and the energy equation, the boundary value problem that will be solved is summarized as: du G d (9) (8) (lo) K K dyk 2U +~ -Wkwk= (k =1K) (12) d d(pykvk) The objetive of the numerial method is find a solution for equations(9-12) with differential equation solver In these equations k denotes k-th speies, Cp is the onstant pressure speifi heat, hk are speies molar enthalpy, ~k are speies moleular weights, Cpk are onstant pressure speifi heat for eah speies and W, are the hemial prodution rates Transport properties, visosity v, thermal ondutivity k and speies diffusion veloities vk are also introdued 15

Boundary Conditions Boundary onditions for temperature, inlet veloity and the miture ompositions at the inlet of fuel and oidizer (plug flow onditions) are speified as follows: Fuel side: =L P~f flf=~, G=O T=Tf, Yk = Ykf, - Oidizer side: =-L pouo uo= 2 G=O, T= TO, Yk = Yk, measured tem- Small symbol of ~ and o denote fuel and oidizer As stated earlier, for these alulations, perature profiles were used instead of solving energy equation Results and Disussion Measured temperature profiles for different water vapor substitutions are shown in Figure 2 The temperature profiles have the same shape eept the peak temperature and the width There is also a small shift in the loation of the peak temperature for the % water ase We believe that the main reasons of this small shift is measurement error andlor hange in properties (thermal and transport) of the oidizer side of the flow with water vapor Interestingly, the maimum temperatures of the flame are inreased with the inrease of water vapor substitution (1914K for O% to 196K for 4% water vapor) and the width of temperature profile is also inreased This means that water vapor whih is added to the flame has other than a physial suppression effet Figure 3 shows veloity profiles with different water vapor substitutions All veloity profiles have nearly the same shape and the same stagnation plane loation (83mm from the fuel eit), beause inerts are substituted with water vapor on the same molar and heat apaity basis Only in the highest temperature zones, the veloity profiles are different beause of different heat release rates and transport harateristis of the miture Figures 4 and 5 show the onentration profiles for major stable speies with % and 3% water vapor substitutions They are all idential eept water onentration beause of water vapor substitutions Figures 6 and 7 eplains the effet of water vapor to the reation of CO and C2 The onentration of C2 is gradually inreased with inrease of water vapor additions while the onentration of CO is dereased The main reation for C2 prodution with CO is as follows: 2 18 ~1 fjoo p :14 b :12? 1 CO+ OH= C2 1 8 A -2,,,,!,,,,,,,,, >, o 5 1 15 2 25 5 1 15 L,,, 2 J, 25 <, FIGURE2 Measured Flame Temperature Profile 5-15 V(m %Wa(e! V(mfsfi 1%water V(onk+ 2G%water V(mw w wh- V(ON51 45%W& i ~GURE 3 Calulated Veloity Profile 1 16

, Therefore, the inrease of C2 with derease of CO means the ative OH radial from water vapor whih is produed in the high temperature flame zone enhanes the reation of CO to C2 Figure 8 shows the differenes in the OH radial onentrations with and without water vapor addition From Figure 8, it is lear that the presene of water vapor in the flame inreases the OH onentration There is, however, a limit to this OH onentration inrease with inrease in water vapor addition Figure 9 shows the onentration of OH radial for various onentrations of water vapor An inrease in the amount of water vapor from 1% to 3%, OH radial onentration inreases, but for the 4 % water vapor addition ase the onentration of OH radial stays almost as same as that of the 3 %ase This result indiates that up to a ertain amount of water addition (in this ase it is between 3~o and 4 %of water vapor addition) the ative OH radial onentration in the flame keeps inreasing, but after this amoun~ inreasing the water vapor onentration does not affet the OH radial onentration (we suspet that this will hange if the 2% is inreased or the flame temperature is inreased) This is the turning point of the hemial enhanement of water vapor to physial suppression effet on the flame Another evidene of the turning point of the hemial effet is shown in Figure 1 Figure 1 shows the onentration of CH3 radial with different water vapor onentrations CH3 is mainly produed by the following reation, ie the first methane oidation reation CH4+H=CH3+H2 8 % 3 ; 6 Q (n 3 g 4 = * ai g 2 8 n N CH, %* a CH, +@ I O2 1,1 - m - Xwo + o 5 1 15 2 25 FIGURE4Major Speies (O% water) 3 ~ 25 aj % 2 5 515 E $ : 1 85!, CCIOY WW = &2Y ww 9 w 4 /Walw 1 :1 K Qo B Xoo,O 7 % ifn * *m s= 9* d 9 5,,,,!, 1 L- 15 2 25 FIGURE6CO Conentration Wau o 5 1 15 2 25 Omz 2u FIGURE5Major Speies (3% water) Uuo ; ~ o*:%wki ~q - CoJaw!bmu J & C%m w g: = $6? % L : L o @ -~ 4 ~ if al!2 o ~ 2 & /: k u +,9 n,, < o 5 1 15 2 2: FIGuR137C2 Conentration 17

In the normal flame, 6% of H atoms reat with CH4 aording to the above reation[6] Therefore an inrease in the prodution of H atoms by water vapor addition inreases the prodution of CH3 and this indiates a more ative flame Figure 1 also shows that as the onentration profile of CH3 is inreased, its width inreases with inrease of water vapor substitution from 1% to 39 However, for the 4% water vapor onentration ase, the width of the CH3 dereases to a value between the 2% and 3% water onentration ases Conlusions Computations of flame struture when water vapor is added to the ounterflow diffusion flame using eperimentally measured temperature profiles are performed for 5 different water vapor onentration ases Maimum temperature and C2 prodution inreased with an inrease of water vapor onentration while CO onentration dereased The onentration of OH radial inreases with water vapor addition This inrease in the OH radial onentration eplains C2 onentration inrease and the maimum flame temperature in$rease The turning point of the hemial effet to the physial effet of water vapor on the flame is found between 3~ and 4% water vapor addition for this 29 ase The onentration profiles of OH and CH3 for various water vapor onentrations further substantiate the eistene of this turning point where the effet of water on the flame hanges from dominant hemial to dominant physial Referenes [1] Muller-Dethlefs, K, Shlader, AF, Combustion and Flame, Vol 27, pp25-215, 1976 [2] Crompton, T, Master thesis at Universi@ of Mihigan, 1995 [3] Lee, KC, An Eperimental Study on Soot Formation and Oidation in Aisymmetri Counterflow Diffusion Flame: PhD thesis at Mihigan State University, pp 91-98, 1991 [4] Kee, RJ, Miller, J A, A Strutured Approah to the Computational Modeling of Chemial Kinetis and Moleular Transport in Flowing Systems, Sandia National Laboratories Report, SAND86-8841, 1992 [5] Kee, RJ Dion-Lewis, G et al, Twenty Seond Symposium (International) on Combustion, pp 1479-1494, The Combustion Institute, Pittsburgh, 1988 [6] Westbrook, CK, Combustion Siene and Tehnology, VOI34DIJ21-225, 1983 6 wj5 OJ %4 = 3 a b $2 v 81 3% waler % I ~ O% waler I ; u:j~:j o 5 1 15 2 25 : ~%% FIGURE8 OH Conentration with & without water 6~ 1 = v OHlw ww!r :5 CHm%water w Xl%Vlala 4 (Jm 6 a+ 4%VIwu fb 84 * *X ~> Distane from the fitel side (m) FIGURE 9OH Conentration 1 C1 $lo%wm o % 2G%Waw f&3%wer A C&4%WW A 9;?& 8 1 12 14 16 FIGURE 1 CH3 Conentration -----,,, 18